Lecture 6 Fluid as skeleton: hydrostatic, hydraulic skeletons
and muscular hydrostats In which we leave solid skeleton and
leverage for skeletons made upon fluid and muscle itself. Animals
with fluid-incompressible skeletons are many: cnidarian polyps,
annelid worms, echinoderms, molluscs (4 differnent major phyla:
Cnidaria, Annelida, Echinodermata, Mollusca). Assigned reading:
Kier W.M. 2012. The diversity of hydrostatic skeletons. Journal of
experimental Biology 215: 1247-1257., Notice Glossary p. 1255 for
terms you may not know: e.g., bulk modulus, mesoglea,siphonoglyph,
etc.
Slide 2
The Introduction of a paper is often the best place to find
useful general information as the writer explains the problem and
what has been done in the past. Animal skeletons serve a variety of
functions in support and movement. For example, the skeleton
transmits the force generated by muscle contraction, providing
support for maintenance of posture and for movement and locomotion.
Also, because muscle as a tissue cannot actively elongate [muscles
cant push], skeletons provide for muscular antagonism, transmitting
the force of contraction of a muscle or group of muscles to
re-elongate their antagonists. In addition, the skeleton often
serves to amplify the displacement, the velocity or the force of
muscle contraction [mechanical amplification]. A wide range of
animals and animal structures lack the rigid skeletal elements that
characterize the skeletons of familiar animals such as the
vertebrates and the arthropods. Instead these animals rely on a
[fluid skeleton]... in which the force of muscle contraction is
transmitted by internal pressure (Kier 2012)
Slide 3
3 main functions of skeleton 1.transmits/translocates force and
is shaped and made of materials that lend themselves to this
translocation (chitin, bone, collagen, resilin...) 2.muscular
antagonism: muscles cant push, they can only pull. So they
typically function in pairs that are antagonists of each other; one
pair member contracts and as it does stretches the antagonist back
to its precontracted dimension. (mandible adductor and abductor are
a good example: often there is a difference in the power/size of
the two antagonists: opening mandibles doesnt require the same
amount of force as crushing food. 3.mechanical amplification:
skeletal leverage via optimal moments of force can increase the
force effect
Slide 4
The exoskeleton of a locust includes cranium, mandible, and the
inflections of mandible cuticle, the two apodemes*; the mandible is
an appendage jointed to the head. Pinnately arranged muscle fibres
originate on the inner cranium and angle downward, converging and
inserting on the apodemes. Muscle contraction pulls on the apodeme
which translocates forces and so moves the mandible. The
contraction of the adductor muscles is antagonized by the abductor
muscles. The moment of force (red perpendicular distance from
apodeme insertion to axis ) is greater for the adductor than the
abductor (blue), because the adductor inserts farther from the axis
of mandible rotation, this lever arm in effect amplifying its
muscle power. *do not forget that the muscles have been omitted;
apodemes dont contract
Slide 5
This insect, Romalea, conspicuous to colour-vision capable
predators, is warningly coloured : it sequesters noxious chemicals.
An adaptation aside
Slide 6
Principles of support and movement (Kier 2012) hydrostatic
skeletons The fluid of hydrostatic skeletons is essentially water ;
it has a high bulk modulus, i.e., resists significant volume
change. Fluids are effectively incompressible. If you stress fluid
(apply a force per unit area to it) its pressure increases without
appreciably changing its volume. When stresses (force per unit
area) are applied to solid skeletons, non-fluids like the
exoskeleton of an insect, the direction of the force matters: pull,
push, or slide stresses give rise to different forces acting in
different dirctions inside the skeleton: tensile, compressive,
shear). But stress applied to a fluid is omnidirectional in effect
: press air into a tire and the tire inflates in any direction it
can get away with (Vogel). With fluid skeletons,contraction of
circular, radial or transverse muscle fibres will decrease
[chamber] diameter, thus increasing the pressure, and because no
significant change in [chamber] volume can occur, this decrease in
diameter must also result in an increase in length. The reverse
occurs to re-expand the diameter and re-elongate the muscle
fibres.
Slide 7
Muscle fibre orientations: Circular, radial and transverse
affect cross-sectional area of a fluid-filled chamber, a hydrocoel:
their contraction causes the fluid skeleton to lengthen and
supports bending. torsion: twisting Muscle fibre orientations:
longitudinal muscle fibres shorten the hydrocoel Some muscle fibres
run helically in muscular hydrostats (cavity less fluid skeleton)
and create torsion, twisting about the long axis of the
structure.
Slide 8
Amplification by hydrostatic skeletons. Consider relationship
between diameter and lengthening. Muscle contraction forces can
still be amplified by hydrostatic skeletons even in the absence of
fulcrums and lever arms. For a constant volume (see Fig. 2 above)
the percentage increase in length, brought about by shortening of
circulars [or radial or transverse fibres], becomes y greater as
diameters decrease.
Slide 9
Collagen an important material in hydrostatic skeletal systems
as crossed fibre helical connective tissue array Collagen is a
important body material, a protein, important consitutent of
fibrous connective tissue; it takes the form of long fibrils
becoming the basis of tendons, ligaments and skin; it is made by
fibroblasts during embryogeny Susan Barker
Slide 10
Connective tissue collagen fibres Dont confuse connective
tissue fibres with muscle fibres: only muscle fibres can contract.
The walls of the hydrostatic chambers are often reinforced with
connective tissue fibres that control and limit shape change. These
fibres are typically arranged in a crossed fibre helical connective
tissue array. Note his language: the fibres are stiff in tension
meaning that when you pull on opposite ends there is negligable
extension. But because they are structured as a helix (a spring if
you like) these relatively inextensible fibres in the wall of a
structure can allow the structure to extend (see echinoderm tube
foot). Elongation and shortening is possible because the pitch of
the helix changes during elongation (the fiber angle, which is the
angle relative to the long axis, decreases) and shortening (the
fibre angle increases)...
Slide 11
Slide 12
Phylum Coelenterata/Cnidaria Hydras, jellyfish, sea anemones,
corals. Radial symmetry. Gastrovascular (GV) cavity (internal
space, filled with seawater) opening via a mouth; no anus, no
assembly-line digestion. Whorl of tentaclesare extensions of body
wall and GV cavity aid in food capture using stinging organelles
(nematocysts). Diploblastic: epidermis and gastrodermis. Mesoglea
layer may give some elasticity The mesoglea ranges from a thin,
noncellular membrane to a thick, fibrous, jelly-like, mucoid
material with or without wandering cells (Barnes). Two structural
types or morphs occur: polyp is sessile; medusa is free swimming.
Very limited development of organs: e.g. siphonoglyph might be
called an organ.
Slide 13
Jennifer Goble Hydrostatic Skeletons Tubastrea DiveGallery Ryan
Photographic
Slide 14
Fig. 5 Kier The body of a sea anemone is a hollow
column...closed at the base...at the top with an oral disc that
includes a ring of tentacles surrounding the mouth and pharynx. By
closing the mouth, the water in the internal cavity the
coelenteron/GV cannot escape, and thus the internal volume remains
essentially constant. The walls of an anemone include a layer of
circular muscle fibres. Longitudinal muscle fibres are found on the
vertical partitions called septa that project radially inward into
the coelenteron, including robust longitudinal retractor muscles
along with sheets of parietal longitudinal muscle fibres adjacent
to the body wall.
Slide 15
Phylum Cnidaria sea anemones, corals, jellyfish etc. With the
mouth closed, contraction of the circular muscle layer decreases
the diameter and thereby increases the height of the anemone.
Contraction of the longitudinal (or R =retractor) muscles shortens
the anemone and re-extends the circular muscle fibres. ...with this
simple muscular arrangement a diverse array of bending movements
and height change can be produced.
Slide 16
ABC News Leeches are also annelids, freshwater, not marine,
specialized for blood feeding. Transverse grooving along leech body
does not represent its ancestral segmentation and is not homologous
with grooving of Nereis. Phylum Annelida segmented worms. Most
species are marine, polychaetes. Annelida have a coelom. Metamerism
and hydrostatic skeletons Coelomate animals Metameres are segments
grouped sometimes into tagmata: a tagma is a series of metameres
specialized for a shared function. Nereis
Slide 17
Animals with a coelom are termed coelomate, animals without one
are acoelomate. What is a coelom? It is defined as a fluid- filled
cavity forming within mesoderm (mesoderm being one of the primary
germ layers of the embryo). To burrow effectively through soil,
searching out softer regions and crevices, going around or under
rocks etc. the worm needs to twist and turn and push its body. For
push you need purchase. For this it has chaetae; producible and
retractable hobnails. chaetae
Slide 18
Schizocoel: splitting coelom formation in annelids Development,
primary germ layers: ectoderm, endoderm, mesoderm (embryonic);
Budding occurs of embryonic tissue behind the trochophore larva, in
a series of segments; bilateral spaces appear in mesoderm and
enlarge until the mesoderm becomes a layer applied against the gut
(endoderm) and the skin (ectoderm). Mesoderm forms mesenteries,
dorsal visceral and ventral. The mesoderm against the forming body
wall differentiates into the circular and longitudinal muscles.
Each somite space expands also to form fore and aft the worms
septum. cronodon.com free-swimming trochophore larva dispersal
stage
Slide 19
What was the primitive function of the segmentation of
annelids? Why partition the coelom? Earthworm is adapted for
burrowing, being able to change body shape locally: a cylindrical
anteriorly pointed probing snout, backed with serial septa-
partitioned hydrostatic skeletal units bounded by muscle: its body
is a flexible digging machine for making its way through soil. o
Annelida: 8800 spp. Triploblastic coelomate bilateria, body cavity
a schizocoel, metamerically segmented, longitudinal and circular
muscles around a hydrostatic skeleton, extracellular digestion in a
straight digestive tract running from anterior mouth to posterior
anus; gut supported by longitudinal mesenteries and septa, ventral
nerve cord with segmental ganglia and anterior brain, circulatory
system high pressure blood in vessels, excretion by metameric
nephidrida. Earthworm Society of Britain Lumbricus castaneus
clitellum Each compartment has its own pair of nephric tubules.
Why?
Slide 20
Slide 21
Slide 22
For cylindrical fluid skeletons at a given pressure, the
stresses in the circumferential direction are twice those in the
longitudinal direction Kier 2012 Septa function in allowing large
lateral forces useful in burrowing